Fix Firefox's bloated UX

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* Prevent massive mega url bar */
#urlbar[breakout],
#urlbar[breakout][breakout-extend] {

  width: 100% !important;
  top: calc((var(--urlbar-toolbar-height) - var(--urlbar-height)) / 2) !important;
  left: 0 !important;
}

#urlbar[breakout][breakout-extend] > #urlbar-input-container,
#urlbar-input-container {
  height: var(--urlbar-height) !important;
  width: 100% !important;
  padding-block: unset !important;
  padding-inline: unset !important;
  transition: none !important;
}

#urlbar[breakout][breakout-extend] > #urlbar-background {
  box-shadow: 0 1px 4px rgba(0,0,0,.05) !important;
  animation: none !important;
}

/*** Prevent massively padded, forever elongated bookmark toolbar menu items (pre FF92 style menu items) ***/
menupopup > menuitem, menupopup > menu {
  padding-block: 4px !important;
  max-width: 20em !important;
}
:root {
  --arrowpanel-menuitem-padding: 4px 8px !important;
}

mega-bar style fix:

• regular url bar
  
• cursor in url bar
  
• typing in url bar
  

menu padding / spacing fix:

• before (FF 92+ style):
  
• after (pre FF 92 style):
  

File Location:

userChrome.css file path:

• mac: /Users/<user>/Library/Application Support/Firefox/Profiles/<profile>/chrome/userChrome.css
• win: %appdata%\Mozilla\Firefox\Profiles\<profile>\chrome\userChrome.css
• linux: ~/.mozilla/firefox/<profile>/chrome/userChrome.css

If the file or chrome folder doesn't exist, create the chrome folder, then create the userChrome.css file inside it and add the above contents, save and restart Firefox.